1. Field of the Invention
The invention relates to optical scanning systems used for laser beam printers, copying machines, and the like. More particularly, the invention is directed to an optical system that scans with a plurality of beams and that has an F.theta. lens system consisting of a plurality of lenses including at least a single cylinder lens.
2. Description of the Related Art
An example of a conventional laser beam printer having an F.theta. lens system that consists of a plurality of lenses including at least a single cylinder lens is shown in FIG. 2. In the conventional example, a light source 1 is constructed of a semiconductor laser or the like. Light injected from such light source 1 passes through a collimator lens 2, passes through a cylinder lens 3 that is inserted so that a beam of light can be injected onto a rotary polygon mirror 4 surface correctly, and is condensed by the rotary polygon mirror 4 only in an sub scanning direction. The constricted beam of light is thereafter deflected and scanned by the rotary polygon mirror 4, passes through F.theta. lens system lenses 5, 6, 7 and forms an image on a photoreceptor 8. It may be noted that the lens 5 is a spherical lens and the lens 6 is a aspherical lens, each of these lenses having different radii of curvature in the sub scanning direction at the central region and at the peripheral region, and that the lens 7 is a cylinder lens.
A sectional view of this optical system as viewed in the sub scanning direction is shown in FIG. 3. In the conventional example, an image is formed on the photoreceptor 8 by appropriately combining a radius of curvature r1 in the sub scanning direction (a radius of curvature in the sub scanning direction of the aspherical lens) and a radius of curvature r2 in the sub scanning direction (a radius of curvature in the sub scanning direction of the cylinder lens). The relationship between r1 and r2 in this case is as shown in FIG. 4. That is, the combinations shown in FIG. 4 would allow an image to be formed on the photoreceptor 8. For such combinations, r2 is related to the magnification in the sub scanning direction as shown in FIG. 5. Thus, r2 is selected so that a desired magnification in the sub scanning direction can be obtained, and r1 is thereafter calculated from r2. It can generally be said that the smaller the magnification, the lower the sensitivity of an optical system. Therefore, the optical system becomes stable and easy to assemble and adjust. Hence, the magnification has heretofore been set to a low value.
On the other hand, if it is assumed that a magnification in the sub scanning direction at the central region of scanning is b and that a ratio of the magnification in the sub scanning direction at the central region of scanning b to a magnification in the sub scanning direction at the peripheral region of scanning b' is a (=b/b'), then the relationship between a and b is as shown in FIG. 6. That is, the smaller the magnification b, the larger the difference between the magnification at the central region and that at the peripheral region with the ratio a becoming smaller than 1. As a result, in one-beam scanning, it happens that the spot diameter becomes smaller toward the peripheral region. However, this phenomenon brings about few conspicuous change in the prints actually made by one-beam scanning. Therefore, this phenomenon has been left unattended.
However, if a plurality of beams are used as the light source under this condition, scanning lines are curved at the peripheral region as shown in FIG. 2 since the beam path itself is curved. Therefore, print pitch error occurs at the peripheral region, which has been a problem.